As a mandrel mill that is equipment for manufacturing seamless pipes, a two-roll mandrel mill has conventionally been used in which two opposed rolling rolls (grooved rolls) are disposed on respective rolling stands, and are arranged alternately so that the rolling direction of rolling roll of a rolling stand is shifted by 90 degrees from that of an adjacent rolling stand. Also, a three-roll mandrel mill has been used in which three rolling rolls (grooved rolls) are disposed on respective rolling stands so that the angle between the rolling directions is 120 degrees, and are arranged alternately so that the rolling direction of rolling roll of a rolling stand is shifted by 60 degrees from that of an adjacent rolling stand.
It has been known that the three-roll mandrel mill is less liable to a problem of plastic deformation such as poor piercing of pipe material than the two-roll mandrel mill.
Therefore, in the case where the three-roll mandrel mill is used, to decrease the equipment size (reduce the number of rolling stands) or to increase the manufacturing capability, it is thought that the working ratio of pipe material per one rolling stand is increased as compared with the case where the two-roll mandrel mill is used.
However, the increase in the working ratio of pipe material per one rolling stand poses a problem that it is difficult for the front edge part of pipe material to bite stably (to secure the roll-biting property). In particular, in the case where the three-roll mandrel mill is used, the change in roll diameter of rolling roll is smaller (the difference between the roll diameter of a groove bottom part and the roll diameter of a flange part is smaller) than the case where the two-roll mandrel mill is used. Therefore, it is more difficult to secure the roll-biting property of the front edge part of pipe material in the case where the three-roll mandrel mill is used.
Also, it has been found that due to the non-steady phenomenon at the time when the pipe material is rolled, as shown in FIG. 4, the cross sections of the pipe edges (the front pipe edge and the rear pipe edge) have a long circumferential length because of being free ends, but excluding the pipe edges, the circumferential length of the rear edge part is short partially. In particular, in the case where the three-roll mandrel mill is used, it is more difficult to secure the circumferential length than the case where the two-roll mandrel mill is used (for example, refer to JP2005-111518A, paragraph 0004). Therefore, in the case where the three-roll mandrel mill is used, it is necessary to compensate for a non-steady portion (a portion in which the circumferential length is short partially) in the rear edge part.